The invention relates to a method for determining the distance between a base station and a mobile object. Further, the invention relates to such a method in an identification system having at least one mobile data memory which is attached to objects and acts as a mobile object for recording object-related status and/or process data. For example, the mobile object could be attached to objects in shipping, transport and/or production systems. The invention also relates to an identification system, a base station, etc., associated with such a method.
Methods for distance determination are known in the conventional art. For example, RADAR is a well known method for determining distances. From a stationary base station, radar waves are emitted and are backscattered by a mobile object, e.g., vehicles, persons, transponders, etc. To determine the distance, known modulation methods based on continuous sinusoidal signals (continuous wave) are preferably used, e.g., the FMCW, LFM or FSK modulation method. A common means to carry out the aforementioned modulation methods is the use of an IQ modulator (quadrature amplitude modulator). By measuring the carrier phase for different frequencies the distance of a mobile object can be measured. Further, similar techniques can be used to determine the speed of a mobile object located in the vicinity can then be determined.
Furthermore, identification systems are known, which have one or more stationary base stations (i.e. read/write devices) which exchange data with mobile data memories acting as mobile objects using a wireless mode of data transmission. The data is transmitted using a data transmission link which is typically radio-based. Systems of this type are used in technical installations in which a plurality of objects or goods must be moved as quickly and freely as possible. The objects that are moved and whose distance is determined can be of a wide variety of types. Examples of such objects are packages in a shipping installation, components in a production plant, luggage in a transport system, etc.
One example of an identification system of this type is described in the ISO 18000-4-MOD3 Standard entitled “Radio-frequency Identification Standard for Item Management—Air Interface.” This standard provides that the read/write device polls for the presence of a mobile data memory within the detection range. For this purpose, it emits an unmodulated HF carrier signal with a defined HF carrier frequency, e.g., 2.45 GHz. A mobile data memory located in the vicinity can passively backscatter this carrier signal modulated with data to the read/write device. The read/write device then processes the received signal.
When operating two or more base station and/or mobile data memories are close to each other care must be taken to avoid undesirable interactions. This situation could occur, for example in adjacent assembly lines. The undesirable interactions could lead to overshoots between the individual base stations and the corresponding mobile data memories. The detection range of the corresponding base station must therefore be restricted. This can be accomplished, for example, by determining the distance between the base station and the corresponding mobile data memory.
Frequencies generated in a conventional manner to determine distances can be subject to significant measurement inaccuracies. This is because the zero phases of the frequencies generated in the base station are not correlated to each other. This is true, for example, if the corresponding frequencies come from different oscillators or are generated by a PLL (Phase Locked Loop). As a result, to get the desired correlation, the signal phases of each frequency must be measured at the transmitter. This requires additional complex circuitry.